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corner can be closed off, Sonia recommended a small electric wall heater, but it’s unlikely to be used much. The separately ducted ventilation system will help distribute conditioned air throughout the house and reduce temperature differentials between rooms. The second floor has a ceiling-concealed ducted indoor unit distributing conditioned air to each of the rooms. This style of air handler was chosen to ensure even distribution of air. Many well-insulated houses with minisplit systems rely on a single head unit placed in the hallway, and the homeowners are comfortable with the temperature differentials that occur when bedroom doors are closed. In the winter, the slightly cooler sleeping temperature in a closed bedroom is preferred by many people; in the summer, the opposite occurs, and bedrooms a few degrees warmer than the set point could feel uncomfortable. The indoor unit and supply ducts to each room are located in a hallway ceiling chase to keep them beneath the air barrier and inside conditioned space. Wall-mounted Tamarack return-air pathways allow airflow back to the single return register in the hallway. MECHANICAL VENTILATION REQUIRED A Zehnder energy-recovery ventilator (ERV) will bring fresh air into the house and remove stale air. As the two airstreams pass each other in an ERV, the conditioned outgoing air tempers the incoming air, reducing the temperature differential the minisplit system must overcome and transferring moisture to the cooler airstream. The ComfoAir 350 ERV, which has an adjustable ventilation rate that ranges from 29 cfm to 218 cfm, was chosen because it met the ProHOME’s ventilation needs at 50% capacity, leaving plenty of capacity to boost the ventilation rate when it’s needed, such as when people are showering. The Passive House–certified Zehnder units are often seen in higher-end builds because of their extremely low electricity consumption and very high energy-recovery efficiency. Although the material costs are at the high end of the range for ventilation equipment, the proprietary ducts, called ComfoTubes, reduce installation time. The 3-in.-o.d. flexible ducts fit inside 2x4 walls and snap easily into diffuser boxes with a simple clip system. The ceiling-mounted supply diffusers can be dialed to the proper flow rate during commissioning. HOT WATER AND COOL AIR First floor: Ceilingmounted cassette In an all-electric house, a heat-pump water heater (HPWH) is the natural choice. Just like air conditioners, HPWHs rely on refrigerant phase changes to transfer heat from the surrounding air to the water in the tank. The by-product is cooler, less humid air. Because they transfer heat rather than creating heat, these water heaters are very efficient, with energy factors as high as 3.39—effectively producing $3.39 worth of energy from $1 of electricity. To overcome the slow recovery rate, an electrical-resistance heating element can be used in place of the heat pump or to supplement it. ProHOME SPONSORS The following manufacturers are supporting the construction of the ProHOME’s high-performance envelope. Visit FineHomebuilding .com/prohome for a complete list of project partners and for more information on the products and materials used in ProHOME. www.finehomebuilding.com OCTOBER/NOVEMBER 2016 77